Further to my last post, I have removed the xen-pciback module from the Dom0 kernel, and reloaded it as

modprobe xen-pciback passthrough=1

I now have the PCI device on the DomU matching the Dom0 Device

usb usb1: SerialNumber: 0000:00:1a.0

instead of 0000:00:00.0

However I now have this error

ehci_hcd 0000:00:1a.0: Unlink after no-IRQ?  Controller is probably using the wrong IRQ.

does this give a clue as to what is going on?

Many thanks

Francis